a. Field of Invention
This invention pertains to a method and apparatus for controlling the warp tension on a weaving loom having multiple beams for feeding the warp threads, and more particularly, for maintaining these warp threads under a constant tension.
b. Description of the Prior Art
Typically, in a weaving loom two devices are used to advance the warp threads and the fabric: a pair of rollers forming a nip engage the woven fabric to advance it by a predetermined amount corresponding to the speed at which the fabric is woven, and one or more beams driven by corresponding D.C. motors for paying off the warp threads. The tension of each warp thread is dependent on the speed of the beams relative to the rollers. If the beams turn faster than the rollers, the warp thread tension is reduced. If the beams turn too slow the warp thread tension will become higher than required.
The warp tension during weaving is very important for certain fabrics because it affects the inner structure of the fabric and certain fabric characteristics. For example, it has been found that for forming fabrics used in papermaking machines, the warp tension must be carefully maintained within a very narrow range during weaving for optimal water drainage, wearability, and minimal marking of the paper.
Initially, all looms employed a single beam for paying off warp threads. The beam was driven by a single motor in a classical analog control loop. However, for fabrics with 20,000 warp threads or more, looms with multiple beams became common, each beam being driven by a corresponding motor. Each motor was provided with an analog control loop, each loop operating in parallel, completely independently of the other loops. However, it was found that tension of warp threads from one beam is affected by the tension of the threads from another beam, and this interdependence tended to unbalance the parallel control loops so that warp tension could not be maintained within the preselected range without considerable tuning by highly trained personnel.
In addition, the tension in each warp thread in a loom varies dynamically during weaving depending on the actual weaving pattern used and this natural variation should be ignored. However, previous analog control loops tried to track these natural variations, causing further variations in the warp tension.